High-precision stable isotope measurements of tungsten and molybdenum in single sample aliquots combined with optimized separation for mixed double spikes

Abstract

A key driver to develop stable tungsten (W) isotope geochemistry is its unique relationship with molybdenum (Mo). Here, we establish a combined double-spike (DS) method for W (¹⁸⁰W–¹⁸⁴W spike) and Mo (⁹⁷Mo–¹⁰⁰Mo spike) to perform simple, efficient, and robust isotope measurements of these two chemically analogous elements in single sample aliquots. Based on previous column chemistry, we optimized two-stage anion-exchange procedures to remove matrix elements, particularly the critical interference of Ta and Hf on ¹⁸⁰W, and to collect sharply separated W and Mo fractions. The obtained recoveries are quantitative for both elements, and their purities are sufficiently high to achieve high-precision measurements comparable to previous DS measurements of individual elements. The reproducibility of our isotope measurements for in-house standard solutions (2SD) was ±0.02‰ for δ¹⁸⁶W and ±0.03‰ for δ⁹⁸Mo. We applied our method to 27 geological reference materials including 10 igneous rocks (AGV-2, JA-3, JR-1, JB-1, JB-1a, JB-2, JB-3, W-2a, TDB-1, and WGB-1), 9 sediments (Nod-A-1, Nod-P-1, JMn-1, JMS-1, JMS-2, CRM7302-a, HISS-1, MESS-4, and PAC-3), and 8 sedimentary and metasedimentary rocks (SDC-1, SDO-1, SBC-1, SCO-1, SCO-2, JSL-1, JSL-2, and IOC-1) to produce a comprehensive data set. The data set confirmed the accuracy of our measurements and expanded the reference materials available for interlaboratory comparisons of δ¹⁸⁶W and δ⁹⁸Mo. The data set also indicates potential pitfalls in sample preparations for particular sample types and shows several variations of W and Mo isotopes possibly related to low-/high-temperature geochemical processes. Our new method, plus the reference data set, will facilitate the development of stable isotope geochemistry for W and Mo.